How to properly diagnose my sound system

I am fairly new around here obviously and very new to the sound game. Recently I brought my equipment to work and it just hasn't been the same since. There is a grounding problem in the outlets here I am pretty sure and I think I might have shocked my amp/dac or headphones. I am not sure to be honest. I only have one pair of headphones so I am kind of at a loss for how to diagnose the problem here. When I got my stuff home There was crazy feedback coming from the headphones. I unplugged the usb cord to the DAC and replugged it and it seemed to be fine. But now I think the quality of sound is significantly lower. The sound is less clear and sounds a little muddy.

Plug the Modi in to a line in on an amp and plug the onboard sound in to another line input on the amp and switch inputs comparing the output. check the sound with the speakers first and then test with headphones.

or

Plug the modi in to the line in on your onboard sound and record. Compare the recorded sound with the original using the onboard sound for playback.

Plug the line out of your onboard sound in to the Magni and test. You could also use an old CD player as a source.

If both tests sound the same maybe the headphones were damaged by the feedback.